The following abbreviations are used in the description below:
3GPP third generation partnership project
ACK/NACK acknowledgement/negative acknowledgement
CSI cyclic shift index
DL downlink
DM RS demodulation reference symbols
e-NodeB Node B of an E-UTRAN system
E-UTRAN evolved UTRAN
H-ARQ hybrid automatic repeat request
LTE long term evolution of 3GPP
MU-MIMO multi-user multiple input/multiple output
Node B base station or similar network access node, including e-NodeB
PBCH physical broadcast channel
PDCCH physical downlink control channel
PHICH physical H-ARQ indicator channel
PMI precoding matrix indicator
PRB physical resource block
UE user equipment (e.g., mobile equipment/station)
UL uplink
UMTS universal mobile telecommunications system
UTRAN UMTS terrestrial radio access network
3GPP is standardizing the long-term evolution (LTE) of the radio-access technology which aims to achieve reduced latency, higher user data rates, improved system capacity and coverage, and reduced cost for the operator. As with any fundamental re-design of a wireless protocol, changing one aspect as compared to an earlier generation system leads to redesign of other portions of the system in order to maximize the advantages to be gained. Specifically, LTE employs the concept of the e-NodeB scheduling its own radio resources within the cell, which gives more flexibility to put available resources to use and also reduces latency in addressing uplink and downlink needs of the various user equipments in the cell. Its most flexible form is dynamic scheduling, where a single scheduling grant sent on a shared control channel grants to one particular user equipment one particular amount of physical resources. This amount of physical resources is constructed of a number of uplink physical resource blocks. The Node B (or its surrogate in the case of relay stations) then must send an ACK or NACK as appropriate to the user equipment once that granted set of UL PRBs passes so the UE can know whether or not it must re-transmit its UL data. LTE sends the ACK/NACK for data received in the UL direction on a special channel (PHICH). The ACK/NACK on the PHICH is made compatible with dynamic scheduling by mapping the UL resource granted to the UE to the particular PHICH where the ACK/NACK is to be, and the development of LTE has seen various proposals for specifics of that mapping.
In general, the HARQ concept includes a forward error detection through CRC (cyclic redundancy check), a feed-back channel for ACK/NACK, and a retransmission mechanism.
So for the case of UL transmission with H-ARQ in LTE, the e-NodeB will transmit the acknowledgement (ACK/NACK) for the UL transmission (at least in case of non-adaptive HARQ) on the PHICH channel. The eNode-B will transmit in the same time the ACK/NACK that is possible for several UEs UL transmissions. The UE needs to know which of those ACKs/NACKs transmitted in the PHICH channel correspond to the UE's own UL transmission.
While dynamic scheduling was noted above, LTE currently aims at using two ways of allocating resources for initial transmission (persistent and dynamic scheduling). As a special case of dynamic allocations, multi user MIMO (MU-MIMO) might be used where the same uplink transmission resources are allocated to two or more users at the same time. For the H-ARQ retransmissions, LTE allows two different ways of allocating resources—either as dynamic scheduling as non-adaptive H-ARQ. It should be noted that the e-Node B might be configured to use only a subset of these options.
One possible solution is shown in a paper designated R1-074588, entitled “PHICH Assignment in E-UTRA” (3GPP TSG RAN1 #51, Jeju, Korea, Nov. 5-9, 2007, by Motorola). R1-074588 describes that for dynamic scheduling (transmission assigned with scheduling grant), the UEs are divided to one or more groups and for each UE group a PHICH group is assigned.
Under current understanding in LTE, a PHICH group consists of physical resources that can at maximum carry 8 ACKs/NACKs in the case where a short cyclic prefix is used; for a long cyclic prefix the number might be less. The UE knows the ACK/NACK resources inside the PHICH group from the CSI of the DM RS, which is signaled to the UE in its UL grant for the corresponding UL transmission. The CSI of the signaled DMRS (or the n_DMRS) can be used to change or identify the PHICH offset as well as the PHICH group (see definitions in section 9.1.2 of 3GPP TS 36.213). This CSI is 3-bits and with these bits the exact ACK/NACK inside the PHICH group can be identified.
This previous approach is also applied for the MU-MIMO case (assigned with scheduling grant). In the MU-MIMO case, two users at different channel conditions are assigned to the same physical (time/frequency) resources and their transmissions can be decoded in the e-NodeB due to those different channel conditions (e.g. different physical locations). To be able to decode MU-MIMO transmissions, separate channel estimations for the two UL transmissions need to be done in the e-NodeB to enable that e-NodeB to have different CSI for both MU-MIMO users.
In the case of non-adaptive HARQ or persistent allocation, the PHICH resources are derived from the used PRBs (e.g., the first PRB index of the allocated UL resources indicates which ACK/NACK resource to use. This can be in any PHICH group. This imposes some scheduling restrictions for the UEs scheduled in the MU-MIMO case and in the dynamic scheduling cases.
The UE needs to know implicitly the PHICH resources in the DL to search for the ACK or NACK that will match its UL transmission, independently of whether the UL transmission is a normal transmission with an allocation grant, a semi-persistent transmission without an allocation grant, or a MU-MIMO or non-adaptive re-transmission.
Reference R1-074588 is extended by reference R1-073409, entitled “MU-MIMO PHICH Assignment for Adaptive and non-Adaptive HARQ” (3GPP TSG RAN1 #50, Athens, Greece, Aug. 20-24, 2007, by Motorola). The MU-MIMO solution in R1-073409 in combination with that of reference R1-074588 gives a more comprehensive solution, but it is more complex and uses more resources than the inventors see as necessary. Moreover, one of the drawbacks of the solution is that it requires grouping of UEs to PHICH groups. This needs additional signaling and introduces restrictions to the scheduler in the e-NodeB in that the scheduler needs to check that only a limited number of UEs from the same PHICH group are scheduled in the same TTI, the limitation being that the maximum number of PHICH in one PHICH group (either 4 or 8 in current understanding of this aspect of LTE).
Other proposals also exists in 3GPP to map the PHICH resource to the UL resource it ACKs or NACKs, however they are seen to address only a subset of the problem rather than all the different UL transmission possibilities that must be ACK'd or NACK'd as laid out above. What is needed in the art is a comprehensive solution to map a UL resource to a DL resource on which the ACK/NACK for that UL resource is sent, regardless of whether the UL resource was dynamically allocated, semi-persistent, or MU-MIMO. Such as solution should be very low in signaling overhead since it will be repeated so often in a practical system.